Geophysical surveys provide data that can provide information about the structure and distribution of rock types and their contents. This information greatly aids searches for water, geothermal reservoirs, and mineral deposits such as hydrocarbons and ores. Most oil companies rely on geophysical surveys to select sites in which to drill exploratory oil wells.
One form of geophysical surveys, electromagnetic (EM) surveys, employ low frequency EM signals that interact with the strata of interest. Such signals may be transmitted from a geophysical source cable towed behind a ship or other surface or subsurface marine vessel travelling across a body of water such as a lake or ocean. The transmitted EM signals propagate through the water and the strata underneath the body of water. As the EM signals interact with the water and strata, the combined resistivity of the water and strata (and, to a lesser extent, the air above the water) causes the EM signals to decay as they propagate away from the source. The resulting electromagnetic field is thus a function of the resistive properties of the water, the sub-bottom strata, and the air. Differences in the strata resistivity along the survey line will translate into minute variations in this electromagnetic field.
Sensors along the length of a geophysical survey cable (which may also be towed by the ship) measure the strength of the electromagnetic field generated by the geophysical source cable, and this acquired data is used to identify and map resistivity contrasts of bodies within the strata (e.g., oil reservoirs). The geophysical survey cable typically obtains electric field measurements with electrodes spaced apart along the length of the cable, and as such, the geophysical survey cable only measures the component of the electrical field parallel to the cable (the “in-line component”). The electrical field component transverse to the survey cable (the “cross-line component”) may offer additional information for mapping the resistivity contrasts, yet previous attempts to measure this component in a marine survey environment have proven inadequate. Given the importance of such maps, and the magnitude of the financial decisions that can depend upon them (e.g., whether to drill an exploration well at a particular location), there is significant incentive to improve their quality.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.